Hereinafter, in the present description, the term “buried cavity” will be used with reference to an empty area (or filled with gas) within a semiconductor body or chip, spaced apart from the two main faces of the body by portions of semiconductor material.
Micromirror devices manufactured using the semiconductor material technology are known and also referred to as MEMS (Micro-Electro-Mechanical System) devices.
These MEMS micromirror devices are, for example, used in portable apparatuses, such as portable computers, laptops, notebooks (including ultra-thin notebooks), PDAs, tablets, and smartphones, for optical applications, for directing, with desired modalities, beams of light radiation generated by a light source.
By virtue of their small size, these MEMS micromirror devices meet stringent requirements regarding bulk, in terms of area and thickness.
For instance, MEMS micromirror devices are used in miniaturized projector modules (so-called picoprojectors), which are able to project images at a distance or to generate desired patterns of lights.
MEMS micromirror devices generally include a mirror element that is mobile, typically with inclination or rotation movements, and is able to direct an incident light beam in a desired way.
For instance, FIG. 1 is a schematic illustration of a picoprojector 9 comprising a light source 1, typically a laser source generating a light beam 2 of three monochromatic beams, one for each basic colour that, through an optical system 3 illustrated only schematically, is deflected by a mirror element 5 towards a screen 6. In the shown example, the mirror element 5 is of a two-dimensional type, controlled so as to turn about a vertical axis A and a horizontal axis B. Rotation of the mirror element 5 about the vertical axis A generates a fast horizontal scan. Rotation of the mirror element 5 about the horizontal axis B, perpendicular to the vertical axis A, generates a slow vertical scan, typically of a sawtooth type.
The obtained scanning scheme is illustrated in FIG. 2 and designated by 7.
In a variant to the system of FIG. 1, the system comprises two micromirrors arranged in sequence along the path of the light beam 2 and each rotatable about an own axis; namely, one is rotatable about the horizontal axis B and the other about the vertical axis A, for generating the same scanning scheme of FIG. 2.
Another application of micromirror systems are 3D gesture-recognition systems. These normally use a picoprojector and an image-acquisition device, such as a camera. The light beam here may be in the visible range, in the invisible range or at any useful frequency. The picoprojector may be similar to the picoprojector 9 of FIG. 1, and the light beam 2 deflected by the micromirror 5 is used for scanning an object in two directions. For instance, the picoprojector may project small stripes on the object; possible projecting or recessed areas of the object (due to the depth thereof) create deformations in the light rays detected by the camera, and these deformations may be detected and used by a suitable electronics connected to the camera in order to detect the third dimension.
In either case, rotation of the mirror element is generally controlled via an actuation system, currently of an electrostatic, magnetic, or piezoelectric type.
For instance, FIGS. 3-5 show a micromechanical mirror structure 10 with purely electrostatic actuation, forming the subject of United States Application for Patent No. 2014/0198366 (incorporated by reference).
The micromechanical mirror structure 10 comprises a first body 11 and a second body 14, for example both of semiconductor material such as silicon, bonded together via a bonding layer (not illustrated), as explained hereinafter.
The first body 11 forms a mobile mass 12 surrounded by a trench 13 and suspended over a cavity, or opening, 15 formed in the second body 14 and having a height (along axis y of a cartesian reference system) smaller than the thickness of the second body 14.
The mobile mass 12 has a central portion 12A, for example, circular in plan view (in the horizontal plane XY), supporting a mirror layer 16. The mirror layer 16 is formed by a material having high reflectivity for the light radiation to be projected, such as aluminium or gold. The mobile mass 12 moreover has supporting portions 12B having an elongated shape and extending on opposite sides with respect to the central portion 12A along axis x of a horizontal plane XY.
The central portion 12A is coupled, at the supporting portions 12B, to anchorages 18, fixed with respect to the second body 14, through elastic elements (springs) 19, of a torsional type, which enable rotation thereof out of the horizontal plane XY.
The elastic elements 19 have a longitudinal extension along axis x and define, along their extension direction, a rotation axis C for the mobile mass 12.
Moreover, the supporting portions 12B of the mobile mass 12 carry, rigid therewith, finger-shaped mobile electrodes 22, which extend in the plane XY on opposite sides of the supporting portions 12B along axis y and within the trench 13.
The micromechanical mirror structure 10 further comprises a fixed portion 23, in the first body 11 and rigid with respect to the second body 14, separated from the mobile mass 12 by the trench 13. The fixed portion 23 carries fixed electrodes 24, also having a finger conformation, facing and combfingered (interdigitated) with the mobile electrodes 22.
First contact pads 25A and second contact pads 25B are carried by respective top surfaces of the fixed portion 23 and of the anchorages 18, for electrical biasing, respectively, of the fixed electrodes 24 and of the mobile electrodes 22.
In use, application of an appropriate potential difference between the mobile electrodes 22 and the fixed electrodes 24 causes torsion of the elastic elements 19 and rotation of the central portion 12A of the mobile mass 12 (and of the associated mirror layer 16) about rotation axis C, according to the desired movement for reflecting an incident light beam towards the outside of the micromechanical mirror structure 10.
The micromechanical mirror structure 10 is currently manufactured as shown in FIGS. 6 and 7 starting from a SOI (Silicon-On-Insulator) substrate.
In detail, FIG. 6, the process comprises bonding a SOI wafer 30 to the second body 14. The SOI substrate 30, in a known way, comprises a first silicon layer 31, an insulating layer 32, and a second silicon layer 33. The first silicon layer 32 operates as handling layer and is thus thick, for example it is 400 μm thick, and the second silicon layer 34, in which the mobile mass 12 of FIG. 3 is formed, has a smaller thickness, for example, 50 μm.
The second body 14 is generally machined before bonding it to the SOI substrate 30. In detail, and in a known manner, for example via deep dry etching, the cavity 15 of FIG. 3 is formed. Furthermore, inside and on top of the second body 14, contact and electrical connection regions (not illustrated) are formed.
The SOI wafer 30 is bonded to the second body 14 through an adhesive layer 36, for example of silicon oxide, glass frit, or other bonding material normally used in MEMS devices. At least part of the adhesive layer 36 may be of a conductive type to enable electrical connection and biasing of the regions formed in the first body 11 of FIG. 3.
Next, FIG. 7, the SOI wafer 30 is thinned, for example via CMP (Chemical Mechanical Polishing) so as to remove the first silicon layer 31 and the insulating layer 32, thus forming the first body 11. Then, for example through selective etching, the second silicon layer 33 is defined to form the mobile mass 12, including the central portion 12A, the supporting portions 12B, and the mobile electrodes 22, as well as the fixed electrodes 23 of FIG. 3 (not visible in FIG. 7).
The mirror layer 16 and the second contact pads 25B are then made on the exposed surface of the second silicon layer 33.
With this solution, the second silicon layer 33 of the SOI wafer 30 has the planarity level desired for forming a micromirror structure, since it is of monocrystalline silicon and thus has high planarity (low roughness) and is thus well suited as a base for the mirror layer 16.
The described process has yielded good results but is relatively costly due to the presence of the SOI wafer which makes difficult to reduce the costs of the micromechanical mirror structure 10 and thus its use in low-cost devices and apparatuses.
There is a need in the art to provide a micromirror device that overcomes the drawbacks of the prior art and in particular may be manufactured at lower costs than the currently.